The invention relates to photoemissive cathodes and particularly to a method of removing trace quantities of cesium and rubidium from a potassium-sodium-antimony photoemissive cathode, which is the cathode of choice in many photomultiplier tubes used in high temperature environments.
In the present context, a trace quantity means an impurity level of a few tens of parts per million. Specifically, it has been determined that all commercially available sodium and potassium chromates which are evaporated to form sodium and potassium vapors, respectively, contain between 10 to 20 parts per million of both cesium and rubidium as impurities. Thus, if one were to attempt to form a potassium-sodium-antimony (bialkali-antimonide) photoemissive cathode using the above described chromates of potassium and sodium, the result would be a multi-alkali antimonide photoemissive cathode comprising predominently potassium, sodium and antimony with cesium and rubidium as unwanted trace inpurities. The presence of cesium and rubidium is thought to create small islands of multi-alkali antimonide photoemissive cathode which have higher dark current or higher noise than the surrounding bialkali antimonide photoemissive cathode comprising potassium-sodium-antimony.
It is known that the photoemissive cathode or photocathode of a photomultiplier tube is adversely affected by high operating temperatures. As the temperature of the tube increases, the dark current of the tube, particularly the thermionic component of the dark current, also increases, thus decreasing the signal-to-noise ratio of the tube. Thermionic emission generally originates from the photocathode itself or from other surfaces in the tube on which alkali materials have been deposited. Typically the photocathode is formed not only on the inside surface of the faceplate but also along the upper sidewall of the tube adjacent to the faceplate. Copending U.S. patent application Ser. No. 546,478, filed by R. D. Faulkner et al. on Oct. 28, 1983, entitled, "ELECTRON DISCHARGE DEVICE HAVING A THERMIONIC EMISSION-REDUCTION COATING," and assigned to the assignee of the present invention, discloses that an indium or induim oxide coating may be deposited along the sidewall to alloy with the constituents of the photocathode to form a high work function layer which is non-photoemissive and which has negligible thermionic emission over the operating temperature range of the tube. The Faulkner et al. copending patent application is incorporated by reference herein for the purpose of disclosure.
The Faulkner et al. patent application, however, offers no solution for lowering the dark current originating from the photocathode on the faceplate. It is known in the art to cool the photomultiplier tube and to reduce the thermionic emission by means of a cryostat; however, with some types of photocathodes, too low a temperature may result in the photocathode becoming so resistive that the photoemission is blocked by a drop in potential across the photoemissive surface. Since the dark current for a potassium-sodium-antimony bialkali photocathode operating at a temperature of 100.degree. C. is about 4.times.10.sup.-16 A/cm.sup.2, whereas the dark current for a potassium-sodium-cesium-antimony multi-alkali photocathode is approximately 4.times.10.sup.-12 A/cm.sup.2, it is clearly beneficial to eliminate the residual impurities such as cesium and rubidium from the potassium-sodium-bialkali photocathode in order to reduce the dark current thereof. However, if lower acceptance levels for cesium and rubidium impurities in commerically purchased potassium and sodium chromates are established, the cost of the materials, and ultimately, the cost of the photomultiplier tube will be increased substantially. Accordingly, it is desirable to find alternative means for eliminating or reducing the trace quantities of cesium and rubidium that are present in the potassium-sodium-antimony photocathode in order to produce a tube having lower dark current.